Make a template - Use a cup larger than the stamp/embellishment you wish to use, trace around it on spare card and cut out.

Using the same cup, place approx half of the cup on the side of the card, try and get it as central on the side as possible. Trace around cup and cut out. Look at photo below for guide of shape.

With the semi circle cut out at one side, now fold in half, trace around edge and cut out for the opposite side. This is now your template.

Place your template on a piece of card. Trace around and cut out. Ink/chalk around the edges.  You can stamp on a design, or write a message see the finished examples at the top of the page for inspiration.

Glue card to patterned paper and cut out, BUT leave a 1cm (approx) gap around the edge.

Get your treat bag (card cello bags are fab), fill with sweets (see below for ideas) and fold the top over. Fold your label in half and place over the top. Staple or hole punch and ribbon the sides.

Here are a few ideas on how to fill your bag...

You could make your own sweets or biscuits - there are some recipes for homemade sweets on the Cooks Journal and also on my blog:
Sweets
Gingerbread Cookies
Buy Sweets the humbugs are ideal for Christmas 
Bath bombs/salts
Hot chocolate kit...eg hot choc sachet, marshmallows, flake
Mixed Nuts
Chocolate raisins

Or maybe make the box into a tissue box - you could cover a tissue box the same and just prise open the bottom to refil.  This is next on my list